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From Rainer Jung <rainer.j...@kippdata.de>
Subject Re: How to align shm in an neat way?
Date Mon, 13 Aug 2012 19:02:21 GMT
On 13.08.2012 19:40, Jeff Trawick wrote:
> On Mon, Aug 13, 2012 at 1:27 PM, Rainer Jung <rainer.jung@kippdata.de> wrote:
>> On 13.08.2012 18:32, Jeff Trawick wrote:
>>>
>>> On Mon, Aug 13, 2012 at 12:30 PM, Rainer Jung <rainer.jung@kippdata.de>
>>> wrote:
>>>>
>>>> Hi,
>>>>
>>>> PR 53040 reveals, that mod_socache_shmcb has an alignment problem. One of
>>>> the three structs mapped into shm contains an apr_time_t member, which at
>>>> least on Sparc is 8 Bytes, whereas for 32 bit builds long is only 4
>>>> Bytes.
>>>>
>>>> Currently everything is aligned for 4 Bytes, so we get bus errors/crashes
>>>> when trying to assign the apr_time_t to an address that is only divisible
>>>> by
>>>> 4 instead of 8.
>>>>
>>>> I can easily reproduce the problem.
>>>>
>>>> A possible solution is to pad the three structures SHMCBHeader,
>>>> SHMCBSubcache and SHMCBIndex to a multiple of 8 Bytes length. For
>>>> Subcache
>>>> and Index this is already true by coincidence, SHMCBHeader needs another
>>>> 4
>>>> Bytes.
>>>>
>>>> I wonder what the right solution is. In the patch
>>>>
>>>> http://people.apache.org/~rjung/patches/mod_socache_shmcb-padding.patch
>>>>
>>>> I hard coded the padding, but I don't really like it, because it breaks
>>>> if
>>>> members are added to the struct. I could add a sizeof() test during
>>>> startup
>>>> or probably even compilation to warn or err, if the padding is wrong.
>>>>
>>>> I see several recipes for alignment using pragmas and attribute, but all
>>>> of
>>>> them are compiler specific.
>>>>
>>>> One could also wrap the struct in a wrapped struct, so that one could use
>>>> the sizeof() of the inner struct to determine the padding of the outer
>>>> struct. That would make the code convoluted.
>>>>
>>>> I checked other parts of the code, but couldn't find a simple solution.
>>>> Any
>>>> hints how to do this nicely?
>>>
>>>
>>> APR_ALIGN_DEFAULT?
>>
>>
>> I think it doesn't solve this problem, does it? It only gives me a need way
>> to round up sizes to multiples of 8 bytes.
>
> It should be possible to use use APR_ALIGN_DEFAULT(sizeof(foo)) in
> place of sizeof(foo), instead of adding explicit padding to the
> structures. Any other pointer arithmetic which doesn't use sizeof(foo)
> would also need to use the macro.

Understood. Unfortunately currently the code doesn't really care about 
alignment. So it just puts structs SHMCBHeader, SHMCBSubcache and an 
array of SHMCBIndex directly after each other in shm. No sizeof() involved.

So yes, option 3 would be to rewrite the code to calculate the 
gaps/padding between the structs using APR_ALIGN_DEFAULT() and adjust 
memory alignment in shm. I wanted to get around changing that part of 
the code ;)

Regards,

Rainer

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